Rotary drill bits are commonly used for drilling boreholes or wells in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. The bit body of a rotary drill bit may be formed from steel. Alternatively, the bit body may be formed from a particle-matrix composite material. A conventional earth-boring rotary drill bit 10 is shown in FIG. 1 that includes a bit body 12 comprising a particle-matrix composite material 15. The bit body 12 is secured to a steel shank 20 having an American Petroleum Institute (API) threaded connection portion 28 for attaching the drill bit 10 to a drill string (not shown). The bit body 12 includes a crown 14 and a steel blank 16. The steel blank 16 is partially embedded in the crown 14. The crown 14 includes a particle-matrix composite material 15, such as, for example, particles of tungsten carbide embedded in a copper alloy matrix material. The bit body 12 is secured to the steel shank 20 by way of a threaded connection 22 and a weld 24 extending around the drill bit 10 on an exterior surface thereof along an interface between the bit body 12 and the steel shank 20.
The bit body 12 may further include wings or blades 30 that are separated by junk slots 32. Internal fluid passageways (not shown) extend between the face 18 of the bit body 12 and a longitudinal bore 40, which extends through the steel shank 20 and partially through the bit body 12. Nozzle inserts (not shown) also may be provided at the face 18 of the bit body 12 within the internal fluid passageways.
A plurality of cutting elements 34 is attached to the face 18 of the bit body 12. Generally, the cutting elements 34 of a fixed-cutter type drill bit have either a disk shape or a substantially cylindrical shape. A cutting surface 35 comprising a hard, super-abrasive material, such as mutually bound particles of polycrystalline diamond, may be provided on a substantially circular end surface of each cutting element 34. Such cutting elements 34 are often referred to as “polycrystalline diamond compact” (PDC) cutting elements 34. The PDC cutting elements 34 may be provided along the blades 30 within pockets 36 formed in the face 18 of the bit body 12, and may be supported from behind by buttresses 38, which may be integrally formed with the crown 14 of the bit body 12. Typically, the cutting elements 34 are fabricated separately from the bit body 12 and secured within the pockets 36 formed in the outer surface of the bit body 12. A bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements 34 to the bit body 12.
During drilling operations, the drill bit 10 is secured to the end of a drill string, which includes tubular pipe and equipment segments coupled end-to-end between the drill bit 10 and other drilling equipment at the surface. The drill bit 10 is positioned at the bottom of a well borehole such that the cutting elements 34 are adjacent the earth formation to be drilled. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit 10 within the borehole. Alternatively, the shank 20 of the drill bit 10 may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit 10. As the drill bit 10 is rotated and weight-on-bit or other axial force is applied, drilling fluid is pumped to the face 18 of the bit body 12 through the longitudinal bore 40 and the internal fluid passageways (not shown). Rotation of the drill bit 10 causes the cutting elements 34 to scrape across and shear away the surface of the underlying formation. The formation cuttings mix with and are suspended within the drilling fluid and pass through the junk slots 32 and the annular space between the well borehole and the drill string to the surface of the earth formation.
Conventionally, bit bodies that include a particle-matrix composite material 15, such as the previously described bit body 12, have been fabricated in graphite molds using a so-called “infiltration” process. The cavities of the graphite molds are conventionally machined with a multi-axis machine tool. Fine features are then added to the cavity of the graphite mold by hand-held tools. Additional clay, which may comprise inorganic particles in an organic binder material, may be applied to surfaces of the mold within the mold cavity and shaped to obtain a desired final configuration of the mold. Where necessary, preform elements or displacements (which may comprise ceramic material, graphite, or resin-coated and compacted sand) may be positioned within the mold and used to define the internal passages, cutting element pockets 36, junk slots 32, and other features of the bit body 12.
After the mold cavity has been defined and displacements positioned within the mold as necessary, a bit body may be formed within the mold cavity. The cavity of the graphite mold is filled with hard particulate carbide material (such as tungsten carbide, titanium carbide, tantalum carbide, etc.). The preformed steel blank 16 then may be positioned in the mold at an appropriate location and orientation. The steel blank 16 may be at least partially submerged in the particulate carbide material within the mold.
The mold then may be vibrated or the particles otherwise packed to decrease the amount of space between adjacent particles of the particulate carbide material. A matrix material (often referred to as a “binder” material), such as a copper-based alloy, may be melted, and caused or allowed to infiltrate the particulate carbide material within the mold cavity. The mold and bit body 12 are allowed to cool to solidify the matrix material. The steel blank 16 is bonded to the particle-matrix composite material 15 that forms the crown 14 upon cooling of the bit body 12 and solidification of the matrix material. Once the bit body 12 has cooled, the bit body 12 is removed from the mold and any displacements are removed from the bit body 12. Destruction of the graphite mold typically is required to remove the bit body 12.
After the bit body 12 has been removed from the mold, the PDC cutting elements 34 may be bonded to the face 18 of the bit body 12 by, for example, brazing, mechanical affixation, or adhesive affixation. The bit body 12 also may be secured to the steel shank 20. As the particle-matrix composite material 15 used to form the crown 14 is relatively hard and not easily machined, the steel blank 16 may be used to secure the bit body 12 to the shank 20. Threads may be machined on an exposed surface of the steel blank 16 to provide the threaded connection 22 between the bit body 12 and the steel shank 20. The steel shank 20 may be threaded onto the bit body 12, and the weld 24 then may be provided along the interface between the bit body 12 and the steel shank 20.